-- | -- Module : Data.ByteString.Search.BoyerMoore -- Copyright : Daniel Fischer -- Chris Kuklewicz -- Licence : BSD3 -- Maintainer : Daniel Fischer <daniel.is.fischer@web.de> -- Stability : Provisional -- Portability : non-portable (BangPatterns) -- -- Fast overlapping Boyer-Moore search of both strict and lazy -- 'S.ByteString' values. -- -- Descriptions of the algorithm can be found at -- <http://www-igm.univ-mlv.fr/~lecroq/string/node14.html#SECTION00140> -- and -- <http://en.wikipedia.org/wiki/Boyer-Moore_string_search_algorithm> -- -- Original authors: Daniel Fischer (daniel.is.fischer at web.de) and -- Chris Kuklewicz (haskell at list.mightyreason.com). module Data.ByteString.Search.BoyerMoore {-# DEPRECATED "Use the new interface instead" #-} ( -- * Overview -- $overview -- ** Changes -- $changes -- ** Deprecation -- $deprecation -- ** Parameter and return types -- $types -- ** Lazy ByteStrings -- $lazy -- ** Performance -- $performance -- ** Complexity -- $complexity -- ** Partial application -- $currying -- ** Integer overflow -- $overflow -- * Functions matchLL , matchLS , matchSL , matchSS ) where import Data.ByteString.Search.Internal.BoyerMoore (matchLL, matchLS, matchSL, matchSS) -- $overview -- -- This module exists only for backwards compatibility. Nevertheless -- there have been small changes in the behaviour of the functions. -- The module exports four search functions: 'matchLL', 'matchLS', -- 'matchSL', and 'matchSS'. All of them return the list of all -- starting positions of possibly overlapping occurrences of a pattern -- in a string. -- $changes -- -- Formerly, all four functions returned an empty list when passed -- an empty pattern. Now, in accordance with the functions from the other -- modules, @matchXY \"\" target = [0 .. 'length' target]@. -- $deprecation -- -- This module is /deprecated/. You should use the new interface provided -- in "Data.ByteString.Search" resp. "Data.ByteString.Lazy.Search". -- $types -- -- The first parameter is always the pattern string. The second -- parameter is always the target string to be searched. The returned -- list contains the offsets of all /overlapping/ patterns. -- -- A returned @Int@ or @Int64@ is an index into the target string -- which is aligned to the head of the pattern string. Strict targets -- return @Int@ indices and lazy targets return @Int64@ indices. All -- returned lists are computed and returned in a lazy fashion. -- $lazy -- -- 'matchLL' and 'matchLS' take lazy bytestrings as patterns. For -- performance, if the pattern is not a single strict chunk then all -- the the pattern chunks will copied into a concatenated strict -- bytestring. This limits the patterns to a length of (maxBound :: -- Int). -- -- 'matchLL' and 'matchSL' take lazy bytestrings as targets. -- These are written so that while they work they will not retain a -- reference to all the earlier parts of the the lazy bytestring. -- This means the garbage collector would be able to keep only a small -- amount of the target string and free the rest. -- $currying -- -- These functions can all be usefully partially applied. -- Given only a pattern the partially applied version will compute -- the supporting lookup tables only once, allowing for efficient re-use. -- Similarly, the partially applied 'matchLL' and 'matchLS' will compute -- the concatenated pattern only once. -- $performance -- -- In general, the Boyer-Moore algorithm is the most efficient method to -- search for a pattern inside a string. The advantage over other algorithms -- (e.g. Naïve, Knuth-Morris-Pratt, Horspool, Sunday) can be made -- arbitrarily large for specially selected patterns and targets, but -- usually, it's a factor of 2–3 versus Knuth-Morris-Pratt and of -- 6–10 versus the naïve algorithm. The Horspool and Sunday -- algorithms, which are simplified variants of the Boyer-Moore algorithm, -- typically have performance between Boyer-Moore and Knuth-Morris-Pratt, -- mostly closer to Boyer-Moore. The advantage of the Boyer-moore variants -- over other algorithms generally becomes larger for longer patterns. For -- very short patterns (or patterns with a very short period), other -- algorithms, e.g. "Data.ByteString.Search.DFA" can be faster (my -- tests suggest that \"very short\" means two, maybe three bytes). -- -- In general, searching in a strict 'S.ByteString' is slightly faster -- than searching in a lazy 'L.ByteString', but for long targets the -- smaller memory footprint of lazy 'L.ByteStrings' can make searching -- those (sometimes much) faster. On the other hand, there are cases -- where searching in a strict target is much faster, even for long targets. -- -- On 32-bit systems, 'Int'-arithmetic is much faster than 'Int64'-arithmetic, -- so when there are many matches, that can make a significant difference. -- -- Also, the modification to ameliorate the case of periodic patterns -- is defeated by chunk-boundaries, so long patterns with a short period -- and many matches exhibit poor behaviour (consider using @indices@ from -- "Data.ByteString.Lazy.Search.DFA" or "Data.ByteString.Lazy.Search.KMP" -- in those cases, the former for medium-length patterns, the latter for -- long patterns; only 'matchLL' and 'matchSL' suffer from -- this problem, though). -- $complexity -- -- Preprocessing the pattern string is O(@patternLength@). The search -- performance is O(@targetLength@\/@patternLength@) in the best case, -- allowing it to go faster than a Knuth-Morris-Pratt algorithm. With -- a non-periodic pattern the worst case uses O(3\*@targetLength@) -- comparisons. The periodic pattern worst case is quadratic -- O(@targetLength@\*@patternLength@) complexity for the original -- Boyer-Moore algorithm. -- -- The searching functions in this module contain a modification which -- drastically improves the performance for periodic patterns. -- I believe that for strict target strings, the worst case is now -- /O/(@targetLength@) also for periodic patterns and for lazy target -- strings, my semi-educated guess is -- /O/(@targetLength@ * (1 + @patternLength@ \/ @chunkSize@)). -- $overflow -- -- The current code uses @Int@ to keep track of the locations in the -- target string. If the length of the pattern plus the length of any -- strict chunk of the target string is greater or equal to -- @'maxBound'::Int@ then this will overflow causing an error. We try -- to detect this and call 'error' before a segfault occurs.